This invention relates generally to heating elements for high-temperature furnaces and, more particularly, to an improved heating element for use in a radiant tube heater in a high-temperature furnace.
The use of radiant tube heaters positioned in high-temperature heat-treating furnaces is, of course, well known. Typically, in the prior art, the interior of a tubular member was gas-fired to heat the tubular member. The tubular member then radiated the heat outwardly into the furnace.
There were various difficulties and problems with gas-fired radiant tube heaters, the most significant of which were the relatively high cost and the low efficiency of gas heating.
The use of electrical heating elements interiorly of a radiant tube is also well known. However, various problems have been encountered with the prior art electrical-heating elements.
Typically, these electrical heating elements include a support structure having a hollow core and an electrical heating assembly. As current is passed through the electrical-heating assembly, the heating assembly radiates heat to the radiant tube, which in turn radiates heat into the furnace.
In the prior art electrical-heating elements, the current-return path from the distal end of the heating element (interiorly of the furnace) to the near end of the heating element (exteriorly of the furnace) was interiorly of the hollow core of the support structure. Thus, any heat generated by current flowing through the conductors in the return path interiorly of the core was substantially of no use in the overall heat generation of the assembly interiorly of the radiant tube.
Yet another problem in the prior art electrical-heating elements is the aging of the electrical conductor. This aging is manifested as oxidation, pitting or corrosion of the conductors. As this condition progresses, small holes appear in the conductors and as these holes become larger and ultimately sever the conductors, the conductors become useless and must be replaced.
Still another problem of the prior art electrical heating elements is lack of stability, i.e., sagging and growth. Sagging is the deviation from the original longitudinal axis and growth is axial elongation, both of which occur during high temperature use of the heating elements.